With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
The initial WLAN technology was able to support the rate of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a 2.4 GHz frequency in accordance with the IEEE 802.11 standard, but the recent WLAN technology can support a maximum rate of 54 Mbps by using Orthogonal Frequency Division Multiplexing (OFDM). Furthermore, in IEEE 802.11, the standards of various technologies, such as the improvements of Quality of Service (QoS), the compatibility of Access Point (AP) protocols, security enhancement, radio resource measurement, a wireless access vehicular environment, fast roaming, a mesh network, interworking with an external network, and wireless network management, are put to practical use or being developed.
Furthermore, in order to overcome limitations to the communication speed that has been considered to be weakness in the WLAN, an IEEE 802.11n standard has recently been regulated as a technology standard. An object of the IEEE 802.11n standard is to increase the speed and reliability of a network and to expand the coverage of a wireless network. More particularly, in order to support a High Throughput (HT) having a maximum data processing speed of 540 Mbps or higher, minimize a transmission error, and optimize the data rate, the IEEE 802.11n standard is based on Multiple Inputs and Multiple Outputs (MIMO) technology in which multiple antennas are used on both sides of a transmitter and a receiver. Furthermore, the standard may use not only a coding scheme for transmitting several redundant copies in order to increase data reliability, but also Orthogonal Frequency Division multiplexing (OFDM) in order to increase the speed.
In a High Throughput (HT) WLAN system based on IEEE 802.11n, a diversity gain and a gain related to an increase of the channel capacity could be obtained by using a Single User (SU) MIMO transmission scheme between an Access Point (AP) and a station (STA). In the SU-MIMO transmission scheme, the degree of freedom of space may be expanded by increasing the number of antennas for transmitting and receiving a radio signal, thereby contributing to the improved performance of a physical layer.
The HT WLAN system has introduced an HT green field PPDU format which may be used in a system including only HT STAs, in addition to the Physical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU) format which supports a legacy STA operated according to the standards of the existing WLAN system. Furthermore, the HT WLAN system supports an HT mixed PPDU format which is a PPDU format designed to support an HT system in a system where a legacy STA and an HT STA coexist.
As the spread of the WLAN is activated and applications using the WLAN are diversified, there is a need for a new WLAN system for supporting the throughput higher than the data processing speed supported by the IEEE 802.11n standard. The next-generation WLAN system supporting a Very High Throughput (VHT) is the next version of the IEEE 802.11n WLAN system and is one of IEEE 802.11 WLAN systems which are recently newly proposed in order to support the data processing speed of 1 Gbps or higher in an MAC Service Access Point (SAP).
The next-generation WLAN system allows a plurality of STAs to access and use channels at the same time in order to efficiently use radio channels. To this end, the next-generation WLAN system supports the transmission of a downlink MU-MIMO scheme using multiple antennas. It is here assumed that the downlink is a link along which data is transmitted from an AP to an STA. The AP may perform Spatial Division Multiple Access (SDMA) transmission in which spatially multiplexed data is transmitted to a plurality of STAs at the same time. The overall throughput of the WLAN system can be increased by distributing a plurality of spatial streams into a plurality of STAs using a plurality of antennas and transmitting data to the STAs at the same time.
Meanwhile, if only downlink MU-MIMO transmission is supported, when STAs transmit frames to an AP, a medium access period is divided for every STA, and the STAs independently transit the frames to the AP. In this case, the improvement of the throughput cannot be expected, as compared with the existing WLAN system, because a common frame transmission scheme or an SU-MIMO transmission scheme is used in the uplink transmission period from the STA. On the other hand, if STAs can transmit frames for traffic processing to an AP at the same time, the overall throughput of the WLAN system may be further improved. In order to further improve the efficiency of a WLAN system, there is a need for a method of transmitting and receiving a data unit which supports uplink MU-MIMO transmission.